Measuring actual displacement at various depths below the surface of an area of soil in response to forces exerted on the surface is conventionally difficult and time consuming. Conventionally, an indicator, such as a flexible film or a layer of powder, is placed between layers of soil. The desired load or force is applied to the soil surface, and the soil is excavated and displacement of the indicators is measured.
Such a process requires that soil be excavated from a hole and the soil placed back in the hole with indicators between layers. In a lab setting, soil and indicators can be layered in a box.
Soils are complex media comprising particulate solids, liquids and gases that can be modeled as visco-elastoplastics. It is known that soil displacement in response to a surface load is greater near the surface, and reduces to zero at some depth below the surface. The distribution of soil deflection beneath the soil surface resulting from a surface applied force has not been accurately measured.
It is often assumed, for practical purposes, that observed surface deflections are cumulative effects of subsurface compactions, however, it is known that soil dynamic characteristics vary with soil composition and depth and are nonlinear. Soil characteristics vary widely among locations.
Conventional technology utilizes load cells and customized piezeoelectric tape sensors, to measure the force exerted at various depths in a soil as a result of a force applied to the soil surface. Deflection of soil beneath the surface is measured typically from the surface impression, assumed to represent cumulative displacement. For many purposes this may be a sufficiently accurate assumption, however for other purposes, a more accurate measurement of subsurface soil displacement would be desirable.
Further, using previously available technology, temporal displacement and force history is not obtainable. Thus the available procedures for displacement measurements are slow, labor intensive, and inaccurate, and there is no known method of measuring soil displacement over time. An improved device for measuring the displacement of soil, and the timing of that displacement, in response to a surface force would be useful, for example, in developing equipment for clearing landmines from an area. A frequent consequence of armed conflicts is the deployment of landmines in soils of affected countries or regions of countries. Large numbers of such landmines remain in large tracts of land after the cessation of armed conflicts. The location of individual landmines within such tracts is almost always indeterminate.
Inadvertent detonation of landmines in former conflict regions causes injury and death daily. Landmines are a major socioeconomic factor that adversely affects countries' abilities to recover from the effects of armed conflict. The presence of landmines at unknown locations in soils disrupts normal trade and commercial activities, access to schools, social services, water and land resources, and other services, and resources sought by people.
Landmines comprise an explosive material and means to trigger detonation of the explosive material. Most landmines are known to be detonated by a certain quantum of downward displacement of an upper structure of a landmine relative to at least one other structure of the landmine. Typically a bias element resists the downward movement of the upper structure, providing a force that must be overcome to cause the displacement necessary to trigger the mine. Such downward displacement can trigger detonation of the explosive material by a variety of known means. Most landmines are designed to be buried in soil such that the downward displacement occurs consequent to the application of a force to the surface of the soil above the deployment site which causes downward displacement of the soil subjected to that force, and translation of that force and such downward displacement of soil to the upper structure of the landmine.
Most deployed landmines are designed to be detonated by either: (a) people (antipersonnel mines); (b) vehicles weighing less than tanks (antivehicle mines); or (c) tanks (antitank mines). Such landmine types differ in that the force required to be overcome to cause a triggering displacement of the upper structure of an antitank mine is greater than the force required to cause triggering displacement of an antivehicle mine's upper structure, which is, in turn, greater that the force required to cause triggering displacement of an antipersonnel mine's upper structure. The range of forces necessary to cause such displacements in landmines of all three types, as produced by a variety of manufacturers, is known.
Usually, forces exerted to the surfaces of soils by people stepping thereon are insufficient to cause detonation of the explosive materials of antitank or antivehicle mines. Also, it is known that antitank and antivehicle mines typically comprise sufficient masses of metallic structures to permit detection of such landmines by known remote metal-detection means whereas many antipersonnel mines comprise little metallic mass and cannot be detected efficiently by such means. Antipersonnel mines typically contain lesser amount of explosive materials that the other landmine types. However, the number of antipersonnel mines deployed far exceeds the number of antivehicle and antitank mines combined, and the preponderance of death, injury, and other loss caused by inadvertent landmine detonations is attributable to antipersonnel mines.
Current efforts to reclaim tracts of land containing or suspected to contain landmines commonly involves detonating landmines by applying force to soil-surface sites sufficient to cause detonation of landmine explosive materials in situ, or sufficient to damage landmine trigger means in situ so as to render the trigger means inoperable. It is preferable that the explosive materials be detonated in situ. Such efforts employ, for example, high-impact flailing hammer mechanisms to strike soil-surface sites.
To detonate a landmine, enough force must be applied to the soil surface to cause translation through the soil of both a sufficient force and a sufficient downward displacement of soil to sufficiently displace the upper structure of the landmine. Thus in compacted soil conditions a force may be translated to the landmine, however the compacted soil does not move, and so the upper structure of the landmine is not displaced, and the landmine remains operational in the soil. Similarly in very loose soils the surface force may be dissipated through the soil prior to reaching the landmine, with the result that there is not sufficient force on the upper structure to overcome the resisting bias force and displace the upper structure. In some cases the loose soil may actually flow around the landmine—there is sufficient displacement but insufficient force. In such conditions a higher force may be required to detonate the landmines than in more typical soil in the same area. For safe and effective antipersonnel mine detonation it would be desirable to determine the forces required to be applied to the surface of soils in particular locations that are sufficient to detonate antipersonnel mines, but insufficient to detonate other mine types. Quantifying the displacement and force pattern within the soil is required and since the applied load or impact from the landmine-detonation device is known to be dynamic, temporal displacement and force patterns should be measured. Compared to the landmine-detonation devices, the feet of a walking person exert a lesser and varying force for a longer period of time. The soil displacement under the soil surface is related to the force exerted and the length of time for which it is exerted on an area of soil surface. Dynamic loading comparisons between the landmine-detonation device and a walking person can be used more accurately to predict the efficacy of the detonation device.
As discussed above, present technology does not provide for convenient measurement of soil displacement and forces over time. Furthermore, such known devices as load cells for measuring force alone are expensive and can be easily damaged by the large magnitude impulse forces exerted by landmine detonating equipment.
Therefore, it would be desirable to have available technology for an in situ measurement of soil temporal displacement and force translation patterns related to site-specific soil deformation characteristics. Ideally, such technology would be robust and of low cost.
An improved device for measuring the displacement of soil would also be useful in measuring the effects of agricultural practices on soil. Heavy equipment operating over the soil surface can cause changes in soil dynamic characteristics, including the formation of subsurface compaction that can affect water retention capacity, penetrability, and other characteristics related to agricultural yields.
Similarly such a subsurface displacement measuring device could be useful in studying other visco-elastoplastic media such as snow, to measure displacement in mountain snowpack, glaciers and the like.